Devices in which an O.sub.2 sensor is provided upstream and downstream of a catalytic converter of a vehicle engine, and an output signal from the two sensors is compared so as to diagnose deterioration of the catalyst, are disclosed in Tokkai Hei 2-30915 published in 1990, and Tokkai Hei 5-106493, Tokkai Hei 5-106494 published in 1993 by the Japanese Patent Office.
In these diagnostic devices, the catalyst is determined to have deteriorated when a ratio of an oscillation frequency of an output signal from the downstream O.sub.2 sensor and an oscillation frequency of an output signal from the upstream O.sub.2 sensor is greater than a predetermined value. These oscillation frequencies are calculated as a number of times the output signal crosses a preset slice level.
These devices comprise one part of an air-fuel ratio feedback controller that, based on the output of the upstream O.sub.2 sensor, feedback control a fuel quantity so that an air-fuel ratio of an air-fuel mixture supplied to an engine reaches a preset target value. The air-fuel ratio feedback control is proportional integral control (PI control), and as a result, the output of the upstream O.sub.2 sensor periodically fluctuates between rich and lean.
U.S. Pat. No. 5,255,662 discloses varying an updating amount of air-fuel ratio feedback correction coefficient according to the difference between the output signal from the downstream O.sub.2 sensor and a predetermined slice level instead of applying a fixed updating amount.
This method of updating the air-fuel ratio feedback correction coefficient reduces the oscillation amplitude of the air-fuel ratio feedback correction coefficient compared to the case where the updating amount is a fixed value.
In this case, the output signal from the O.sub.2 sensor fluctuates with a smaller period than when the updating amount is a fixed value, and the number of times that the output signal from the downstream O.sub.2 sensor cuts across the slice level therefore increases. This leads to an increase of the aforesaid frequency ratio. As a result in this type of air-fuel ratio controller, when the aforesaid catalyst deterioration diagnosis was performed, it may be incorrectly determined that the catalyst had deteriorated when in fact it was operating normally.